HYDROGEN-BONDING CLUSTER FORMATION AND HYDROPHOBIC SOLUTE ASSOCIATIONIN AQUEOUS-SOLUTION OF ETHANOL

Hydrogen bonding (HE) cluster formation of ethanol and water molecules in diluted aqueous solutions (x(A) less than or equal to 0.03) is inv estigated on the bases of infrared (LR) absorption spectroscopy, mass spectrometric analyses of the clusters, and X-ray diffraction measurem ent. With increasing alcohol molar fraction (x(A)) up to 0.03, IR spec tra showed the intensity decrease on the high-frequency side of the O- H stretching band of water. The s.i(s) curve at x(A) = 0.02 showed str iking resemblance to the reported pure water curve at 1 kbar. The X-ra y radial distribution function exhibited the decrease of linear hydrog en bonds (LHB) at 2.85 Angstrom and the new peaks at 3.3 and 3.8 Angst rom. The new peaks correspond to the O-O distance of an angular hydrog en bond and the distance from an ethyl carbon to a water oxygen of the hydrogen-bonding cage, respectively. Mass spectra of the clusters rev ealed the dimerization of ethanol in a very diluted solution (x(A) app roximate to 0.001) at 35 degrees C. The clusters isolated from an x(A) = 0.02 solution were composed of water and ethanol molecules with an average molecular number ratio (water / ethanol) of 2 (+/- 1). This in dicates that the solute-solute association is highly preferable even a t such a diluted concentration. A hydrophobic core structure composed of coherent ethyl groups with a strong water hydrogen-bonding cage is presented to explain the observed results.